CMOS Image sensors (CIS) suffer from the issue that deeply scaled sub-micron Complementary Metal Oxide Semiconductor (CMOS) processes are required to realize small pixels that can compete with charge-coupled device (CCD) pixel sizes. In general, as the CMOS processes scale to smaller dimensions, the details of the process integration and structure change, and the pixel performance degrades. Some examples of this are shallow trench isolation, and heavily doped retrograde wells. Both are necessary to build deep sub-micron CMOS devices, but both have adverse effects on dark current for pixels. As a result, much work has to be done to re-integrate and re-optimize the photodetector and pixel into each new deep sub-micron CMOS technology node.
Designers, however, face a trade-off with respect to the design and manufacture of sub-micron CMOS devices. Designers can either maintain pixel image quality by not moving to more scaled CMOS processes, which results in a lower fill factor for smaller pixels, or move to a smaller design rule process to achieve small pixels, which results in a need to re-integrate and re-engineer the photodetector to obtain acceptable image quality.
One solution to these issues is to build the photodetector separately from the CMOS circuits. The image sensor, for example, can be built on different wafers, and the wafers joined together using three-dimensional integration or wafer level interconnect technologies. U.S. Pat. No. 6,927,432 fabricates an active pixel sensor using two semiconductor wafers. One wafer, the donor wafer, includes the photodetectors while another wafer, the host wafer, includes an interconnect layer and electrical circuits for in-pixel signal operations and read out of the photodetectors. Pixel interconnects directly connect each photodetector on the donor wafer to a respective node or circuit on the host wafer.
Although this approach separates the processing of the photodetector and circuits, it degrades photodetector performance due to the direct contact or connection with the photodetector. Specific examples of such performance degradation include, but are not limited to, increased dark current due to damage from the contact etch process, increased metallic contamination in the photodetector leading to point defects, and high dark current due to being connected to a highly doped ohmic contact region.